Cardiac pacemakers stimulate the heart with electrical impulses to induce a heartbeat. Pacemakers may also sense the condition of the heart so that stimuli may be applied in an appropriate manner. Pacemakers have been developed which respond to detected physiological parameters to provide variable pacing rates which more nearly approximate the physiologic requirements of a patient. Such parameters as pH, oxygen saturation, intracardiac temperature and other parameters are sensed alone or in combination to provide information concerning the physiologic demands of the patient.
Cardiac pacing leads designed to sense such parameters, other than merely sensing electrical signals and pacing the heart, are typically of special design. For example, a pacemaker lead with a temperature sensor is disclosed in U.S. Pat. No. 4,726,383, which requires implantation of a specific lead containing a thermistor transducer. A specialized lead with a pressure sensor or an accelerometer is disclosed in U.S. Pat. No. 4,666,617 and again requires a specific lead with a built-in sensor. U.S. Pat. No. 4,690,143 discloses a pacemaker having a lead which can generate electrical power piezoelectrically from the movement of the lead. Such a lead requires a piezoelectric element built in along the length of the lead.
These designs require a specialized lead for sensing special parameters. Consequently, if a pacemaker responsive to such parameters is to be implanted into a patient, the lead must correspond to the given pacemaker. It is not unusual for patients to require or want replacement pacemakers, either because the batteries on their prior pacemaker are expended or to gain new and improved features. In such situations, a lead may already be implanted in the heart. Such a lead will have achieved a reliable fixation. Moreover, it is a known phenomenon for a newly implanted lead to have an initial period of increased impedance resulting, apparently, from inflammation or the response of the heart to a new foreign body in tissue. Leads that have stabilized in position can, therefore, be expected to have superior impedance characteristics. It may be impossible or risky to remove a lead because of fixation to heart tissue. A physician, therefore, may want to avoid removal of a lead which is already successfully implanted in the heart.
Moreover, specialized sensors such as pressure or accelerometer sensors tend to have a finite life and the sensors may fail over time. It is desirable, therefore, that the sensor should be replaceable, without replacing the entire lead.
It is also possible to combine pacing, antitachycardia, defibrillation, or other electrical therapy with a chemical or drug therapy. Both therapies may be delivered to the heart through a single lead.
With the foregoing in mind, it is an object of my invention to provide a lead system wherein different sensors can be selected for different applications.
It is also an object of my invention to provide a lead system wherein the sensor can be replaced without explanting the entire lead.
A further object of my invention is to provide apparatus whereby a sensor can be added to an existing, implanted lead without removing the lead from the patient's body.
Yet another object of my invention is to provide a lead which can deliver both electrical and chemical therapies to the heart of the patient.